The present invention relates to circuits used in power supplies and amplifiers, and more particularly to the use of pulse width modulation in power supply and amplifier circuitry.
Power supplies and amplifiers have previously been designed to incorporate pulse width modulation in order to achieve higher operating efficiency, lower part count and, hence, higher reliability. In such circuit designs, a pulse width modulated signal is developed in response to an input reference signal which may be a rapidly varying signal, such as an audio input, or a slowly varying reference voltage in the case of regulated power supplies and control system circuitry.
In audio amplifiers, for example, it is well known that pulse width modulation (PWM) can be effectively used in a Class D amplifier configuration to control the amount of power delivered to an output load, such as a loudspeaker: G. F. Turnbull, M.Sc., and J. M. Townsend, M.Sc., "Efficiency Considerations in a Class D Amplifier", Wireless World, April 1967, pp. 154-158; K. C. Johnson, M.A., "1: Class D Principles Analysed", Wireless World, December 1967, pp. 576-580; Tadao Suzuki, "Application of Vertical FET for Pulse Width Modulation Audio Power Amplifier", presented at 55th Convention of Audio Engineering Society, Nov. 1, 1976, Preprint No. 1158; Brian E. Attwood, "Very High Fidelity Quartz Controlled PWM (Class D) Stereo Amplifiers for Consumer and Professional Use", presented at 59th Convention of Audio Engineering Society, Feb. 28-Mar. 3, 1978, Preprint No. 1331.
In such prior designs, a source of AC line power is converted by rectification and filtering to DC and a controlled amount of this DC power is switched to the loud-speaker connected as the output load. The DC is applied to the load cyclically by switching transistors or other solid state switches operated at a variable duty cycle in response to the audio input signal. The duty cycle of the DC to load switching is controlled by converting the input signal into a PWM signal such that the width of each pulse in a cycle varies according to the amplitude of the input signal. The duty cycle (duration during each cycle that the DC is connected across the load) is then varied according to the widths of the pulses of the PWM signal. The average energy or power delivered to the load thus changes according to the amplitude of the input signal. Since the voltage signal applied across the load includes the switching transients of the modulation cycle, a low pass filter is usually employed to pass just the audio or other input signal information and exclude the relatively higher frequency switching transient components.
Similar circuits are used in servo control systems in which the input is a varying control signal and the output load is, for example, an electromechanical device.
For other applications, such as a regulated power supply output, the input signal is a reference voltage and the output load is the electrical appliance, circuit or other powered impedance connected to the power supply. It is common in the case of such regulated power supplies to convert an available AC line power, such as a 120 volt-400 Hz line power, common, for example, in the aircraft industry, to DC using rectification and filtering. Then the resulting DC is cyclically connected across the load in accordance with a PWM control signal that varies in pulse width in accordance with the reference signal. The average power delivered to the load is thus regulated by the reference which may be a feedback error signal responsive to the load power.
In all of these applications, power supply regulation, audio and control signal amplification, the PWM configuration significantly reduces power consumption as well as minimizing the number of electrical components, hence increasing the reliability of such designs.
Examples of PWM designs for power supplies and control systems are found in the following representative collection of prior art patents and literature: Linear/Switchmode Voltage Regulator Handbook, Second U.S. Edition, MOTOROLA INC., 1982, pp. 109-110; George Chryssis, High Frequency Switching Power Supplies, "Switching Regulator Control Circuits", McGraw-Hill Book Co., 1984, pp. 134-137; Andrew Zaderej, U.S. Pat. No. 4,626,746 for POWER CONTROL CIRCUIT, issued Dec. 2, 1986; Billy Harold Hamilton, Helmut Wilhart, U.S. Pat. No. 3,735,235 for DC TO DC CONVERTER WITH VOLTAGE REGULATION FEEDBACK LOOP------, issued May 22, 1973.